The Advanced Television Systems Committee (ATSC) published a Digital Television Standard in 1995 as Document A/53, hereinafter referred to simply as “A/53” for sake of brevity. Annex D of A/53 titled “RF/Transmission Systems Characteristics” is particularly incorporated by reference into this specification. So is Section 5.6.3 titled “Specification of private data services” from Annex C of A/53. In the beginning years of the twenty-first century efforts have been made by some in the DTV industry to provide for more robust transmission of data over broadcast DTV channels without unduly disrupting the operation of so-called “legacy” DTV receivers already in the field. The operation of nearly all legacy receivers is disrupted if ⅔ trellis coding is not preserved throughout every transmitted data field. Also, the average modulus of the signal should be the same as for 8VSB signal as specified in the 1995 version of A/53, so as not to disrupt adaptive equalization in legacy receivers using the constant modulus algorithm (CMA).
Another problem concerning “legacy” DTV receivers is that a large number of such receivers were sold that were designed not to respond to broadcast DTV signals unless de-interleaved data fields recovered by trellis decoding were preponderantly filled with (207, 187) Reed-Solomon forward-error-correction (R-S FEC) codewords of a specific type or correctable approximations to such codewords. Accordingly, in order to accommodate continuing DTV reception by such legacy receivers, robust transmissions are constrained in the following way. Before convolutional byte interleaving, data fields should be preponderantly filled with (207, 187) R-S FEC codewords of the type specified in A/53.
In 2006 engineers of Samsung Electronics Co., Ltd. proposed introducing further-coded ancillary data into adaptation fields of the 187-byte MPEG-2-compatible data packets included in the 207-byte data segments of the 8VSB DTV broadcast signals used in the United States. This scheme, called “A-VSB”, was championed because the packet decoders in legacy DTV receivers could readily disregard the further-coded ancillary datastream. This provides a form of backward compatibility in which those legacy DTV receivers can still receive a principal datastream transmitted in the payload data fields of the 187-byte MPEG-2-compatible data packets. There is no backward compatibility, however, in the sense that legacy DTV receivers can usefully decode the information content in the further-coded ancillary datastream. Nominally, the code rate of A-VSB is one-half the code rate of ordinary 8VSB in its less robust form or one-quarter the code rate of ordinary 8VSB in its more robust form. A-VSB uses serially concatenated convolutional coding (SCCC) that incorporates the ⅔ trellis coding characteristic of 8VSB DTV signals as the inner convolutional coding, so SCCC can be accomplished by essentially just halving the ordinary code rate for 8VSB. This form of SCCC is not systematic; that is, the data do not appear in their original form in the signal resulting from the ⅔ trellis coding. The ancillary data are randomized and then subjected to (207, 187) R-S FEC coding, and the resulting (207, 187) R-S FEC codewords are convolutionally interleaved before being subjected to the outer convolutional coding and subsequent bit interleaving. U.S. patent application Ser. No. 11/416,245 of Jeong et alii published 19 Jul. 2007 with publication No. 2007-0168842 and titled “Transmitter and system for transmitting/receiving digital broadcasting stream and method thereof”, which is incorporated by reference, describes in considerable detail practices used in A-VSB.
In 2007 Samsung engineers proposed adapting their A-VSB transmission system for mobile reception by DTV receivers that are carried by fast-moving vehicles such as automobiles, buses or railroad passenger cars. Such reception is disrupted by momentary “deep fades” or drop-outs in received signal strength as the vehicle moves through underpasses or passes large buildings blocking the transmission path. To help a mobile DTV receiver withstand these momentary deep fades, the Samsung engineers introduced an outer byte interleaver after the encoder used to generate the (207, 187) R-S FEC codewords supplied for serially concatenated convolutional coding. This outer byte interleaver spread the successive bytes of each (207, 187) R-S FEC codeword apart so far that fewer of them would be lost during a momentary deep fade. Hopefully, so few bytes would be lost in each (207, 187) R-S FEC codeword that the Reed-Solomon decoding apparatus in a DTV receiver designed for mobile reception would be able to correct the codeword and restore the missing bytes.
In 2007 LG Electronics proposed introducing further-coded ancillary data into the 184-byte payload fields of the 187-byte MPEG-2-compatible data packets included in the 207-byte data segments of the 8VSB DTV broadcast signals used in the United States. The LG Electronics system, commonly referred to as “MPH”, subjected ancillary data to preliminary two-dimensional coding procedures designed to help a mobile DTV receiver withstand momentary deep fades. This preliminary two-dimensional coding comprised transversal Reed-Solomon (TRS) coding of ancillary signals within a Reed-Solomon frame extending over 968 milliseconds or twenty 8VSB frame times. This preliminary two-dimensional coding further comprised periodic cyclic-redundancy-check coding used for locating byte errors for the TRS.
Another known technique for overcoming temporary fading is called “staggercasting”, a variant of which Thomson, Inc. has proposed be used in robust 8VSB transmissions. Staggercasting can also overcome certain types of intermittent radio-frequency interference. Staggercasting communications systems transmit a composite signal including two component content-representative signals, one of which is delayed with respect to the other. The composite signal is broadcast to one or more receivers through a communications channel. At a receiver, delayed response to the earlier transmitted component content-representative signal supplied from a buffer memory is contemporaneous in time with the later transmitted component content-representative signal. Under normal conditions, the receiver detects and reproduces the content of the later transmitted signal as soon as it is received. However, if a drop-out in received signal strength occurs, then the receiver detects and reproduces the content of the earlier transmitted signal as read from buffer memory. If the delay period and the associated delay buffer are large enough, then fairly long drop-outs in received signal strength can be overcome. This capability not only requires a severalfold increase in the amount of memory required in a receiver; it halves the effective code rate of the transmission.
The inventor perceived that the processing of “soft” decisions in turbo decoding allows a more sophisticated approach to be taken for the reception of staggercasting. “Soft” decisions concerning the contents of an earlier transmitted turbo codeword and concerning the contents of a later transmitted repeat of the earlier transmitted turbo codeword can be analyzed for selecting which of corresponding portions of the two turbo codewords as received is more likely to be correct. The selection procedure can synthesize a turbo codeword that is more likely to be correct than either of the turbo codewords from which the parts of the synthesized turbo codeword are drawn. The synthesized turbo codeword can then be subjected to turbo decoding and R-S decoding procedures.
The inventor subsequently invented a “punctured staggercasting” in which parallel concatenated convolutional coding (PCCC) was dissected for transmission. Data and the parity bits for one of the two convolutional codes used in the PCCC are transmitted at an earlier time in “punctured staggercasting”. Subsequently, at a later time, the data are retransmitted together with the parity bits for the other of the two convolutional codes used in the PCCC. In the receiver “soft” decisions concerning the originally transmitted data and “soft” decisions concerning the re-transmitted data are compared, and a best estimate of the data is developed for PCCC decoding. Deep fading conditions that prevent successful reception of one of the transmissions may not affect the other transmission severely enough to prevent its being successfully received.
Some time after this, the inventor realized that this “punctured staggercasting” concept can be applied to SCCC of the types used in A-VSB and in MPH. Of especial interest is the application of “punctured staggercasting” to SCCC in which the earlier transmission and the later transmission are each at a code rate that is nominally one half that of ordinary 8VSB. Overall, a code rate that is nominally one quarter that of ordinary 8VSB results, and AWGN performance is expected to be similar to that of previously proposed A-VSB or MPH signals having a code rate that is nominally one quarter that of ordinary 8VSB. However, except when SNR is very low for both transmissions of the “punctured staggercasting” signals, reception should be possible. Deep fading conditions can be tolerated that would not be successfully received using the previously proposed A-VSB or MPH signals having a code rate that is nominally one quarter that of ordinary 8VSB.
A problem receivers for staggercast SCCC or PCCC DTV signals are prone to is difficulty in changing channels quickly owing to the latent delay involved in combining the earlier transmitted signals with later transmitted signals. The inventor discerned that this problem can be alleviated when strong signals are received. When a channel is initially tuned to, only the later transmitted words of the staggercast SCCC or PCCC are decoded until earlier transmitted words that have been temporarily stored for combining with the later transmitted words of the staggercast SCCC or PCCC become available. This approach works best if earlier transmitted words and later transmitted words of the staggercast SCCC or PCCC are interleaved in time.